1. Field of the Invention
The present invention relates to an ultra thin type epitaxial wafer and a light emitting diode.
2. Description of the Related Art
A light emitting diode is a device which directly converts electric energy into light where a forward current is applied to a p-n junction of semiconductor. III-V group compound semiconductors are frequently used as materials of light emitting diodes because they have a band gap corresponding to a wavelength of infrared to ultraviolet light. Among these, gallium phosphide (GaP) light emitting diode devices emitting red to green light., and gallium arsenide (GaAs) light emitting diode devices emitting infra-red to yellow light are widely used.
FIG. 10 shows an exemplary structure of conventional GaP light emitting diode. In this structure, an n-type GaP epitaxial layer 13 (also referred to simply as xe2x80x9cbuffer layerxe2x80x9d hereinafter) having a thickness of 100 xcexcm and a carrier concentration of 8xc3x971016 cmxe2x88x923 is grown on an n-type GaP single crystal substrate 12 having a thickness of 280 xcexcm and a carrier concentration of 1xc3x971017 cmxe2x88x923, and a light emitting region composed of an n-type GaP epitaxial layer 14 having a thickness of 20 xcexcm and a carrier concentration of 6xc3x971016 cmxe2x88x923, p-n junction 15, and p-type GaP epitaxial layer 16 having a thickness of 20 xcexcm and a carrier concentration of 1xc3x971017 cmxe2x88x923 stacked in this order is provided thereon.
By forming n-electrodes 11 and a p-electrode 17 are formed on the above structure, a GaP light emitting diode 20 having a total thickness of 420 xcexcm of the compound semiconductor single crystal substrate 12 and the epitaxial layers 13-16 can be obtained.
The thickness of conventionally used buffer layer 13 is 100 xcexcm at the utmost. This is because the buffer layer 13 having a thickness of about 100 xcexcm can afford sufficient light emission intensity as light emission intensity conventionally required.
This buffer layer 13 is required to obtain high light emitting intensity partly for the following reason. That is, as for currently available materials for the GaP single crystal substrate 12, dislocations are generated at a high density, i.e., around 5xc3x97104 cmxe2x88x922 to 1xc3x97105 cmxe2x88x922, and their purity is also low. Therefore, the epitaxial layer may have bad quality at a portion near the GaP single crystal substrate 12 grown at an early stage of the growth, and such an epitaxial layer as it is cannot afford high light emitting intensity. Accordingly, the buffer layer 13 is provided as a layer for buffering between the epitaxial layers 14-16, which constitute the light emitting region, and the GaP single crystal substrate 12.
Another reason why the buffer layer 13 is required arises from the fact that the light transmissivity of the GaP single crystal substrate 12 is inferior to that of the buffer layer 13, which is an epitaxial layer. FIG. 11 represents light transmissivity of the GaP single crystal substrate 12 and the buffer layer 13 plotted to the. wavelength of incident light. For example, about 5% of difference in the light transmissivity is observed around 570 nm, which corresponds to a wavelength of yellow green light emission commonly used in GaP light emitting diodes.
Therefore, a thinner compound semiconductor single crystal substrate (also referred to simply as xe2x80x9csingle crystal substratexe2x80x9d) has been used to suppress the degradation of the light transmissivity. As a means for obtaining a thinner single crystal substrate, a method utilizing a thin single crystal substrate for epitaxial growth can be mentioned first. However, according to such a method, more single crystal substrates are broken during the epitaxial growth process, as the single crystal substrates become thinner, and therefore the productivity is markedly decreased.
Judging from the current state of the art, the best way for obtaining a thinner single crystal substrate is scraping the main back surface of single crystal substrate by, for example, lapping after the epitaxial growth. However, when a single crystal substrate is scraped by lapping or the like to an extent that the remaining thickness of the substrate becomes 10 xcexcm or less, a part of the single crystal substrate may drop out so that the epitaxial layer is exposed, because of bad precision of the lapping process, nonuniform growth of the epitaxial layer or the like.
The conventionally used single crystal substrate 2 has a carrier concentration not less than 1xc3x971017 cmxe2x88x923, and allows ohmic contact with the electrode 1. On the other hand, in the production of light emitting diodes, the carrier concentration of the epitaxial layer which is provided directly on the single crystal substrate has conventionally been made smaller than 1xc3x971017 cmxe2x88x923 in order to realize high luminous intensity.
Therefore, if a part of the main back surface of the single crystal substrate drops out, and the epitaxial layer is exposed, an ohmic electrode may not be formed at the exposed area of the epitaxial layer, and problems concerning optoelectronic characteristics may be arisen, for example, the forward voltage may become high. Accordingly, the conventional single crystal substrate could not be made thinner than 10 xcexcm as the remaining thickness.
Further, with the recent increasing demand for smaller and thinner electronic products and the like, it also becomes necessary to make light emitting diodes thinner. In the conventionally used light emitting diodes, the total thickness of the compound semiconductor single crystal substrate and the epitaxial layer was typically 250 xcexcm to 450 xcexcm.
The present invention has been accomplished in view of the problems of the prior art mentioned above, and its object is to provide an ultra thin type light emitting diode where generation of ohmic electrode failure is suppressed, and a epitaxial wafer for the foregoing light emitting diode.
In order to achieve the aforementioned object, the present invention provide an epitaxial wafer comprising an epitaxial layer formed on a main surface of a compound semiconductor single crystal substrate, characterized in that the epitaxial layer on the main surface is exposed in a back surface of the compound semiconductor single crystal substrate, and an exposed portion of the epitaxial layer has a carrier concentration of 1xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923.
An epitaxial wafer where the epitaxial layer on the main surface. is exposed in the back surface of the compound semiconductor single crystal substrate, and an exposed portion of the epitaxial layer has a carrier concentration of 1xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923 as defined above can suppress the generation of ohmic electrode failure, because such an epitaxial wafer does not cause problems that an ohmic electrode cannot be formed at the exposed portion, the forward current becomes high and the like, even when the main back surface of the compound semiconductor single crystal substrate was scraped by lapping or the like to such an extent that the epitaxial layer on the main surface is exposed.
In the above epitaxial wafer, the carrier concentration of the epitaxial layer exposed in the back surface of the compound semiconductor single crystal substrate is preferably from 1xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923 within a range of at least 5 xcexcm from the main surface of the compound semiconductor single crystal substrate along the epitaxial layer growing direction.
If the carrier concentration of the epitaxial layer exposed in the back surface of the compound semiconductor single crystal substrate is from 1xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923 within a range of at least 5 xcexcm from the main surface of the compound semiconductor single crystal substrate along the epitaxial layer growing direction, a sufficient carrier concentration for forming an ohmic contact at the exposed portion of the epitaxial layer can be maintained, even if there are fluctuation of lapping process and nonuniformity of the epitaxial layer growth.
Further, in the above epitaxial wafer, the epitaxial layer exposed in the back surface of the compound semiconductor single crystal substrate is, for example, a buffer layer having such a thickness that dislocation density should be constant along the epitaxial layer growing direction, and preferably formed as a light emitting layer on the buffer layer.
When the epitaxial layer exposed in the back surface of the compound semiconductor single crystal substrate is a buffer layer having such a thickness that dislocation density should be constant along the epitaxial layer growing direction, and formed as a light emitting layer on the buffer layer, the reduction rate of light emitting intensity caused by influence of dislocation density also becomes constant, and therefore difference of light emitting intensity among individual light emitting diodes can be reduced.
Furthermore, according to the present invention, the compound semiconductor single crystal substrate preferably has a thickness of 10 xcexcm or less. When the substrate satisfies this requirement, the degradation of light transmissivity due to the compound semiconductor single crystal substrate can be suppressed, and the thickness of the light emitting diode as a whole can also be made markedly smaller. Therefore, an ultra thin type light emitting diode with high light emitting intensity can be realized.
Further, the aforementioned buffer layer preferably has a thickness of 120 xcexcm to 250 xcexcm. When the buffer layer satisfies this requirement, a range where the dislocation density is constant along the epitaxial layer growth direction becomes larger in the buffer layer, and therefore light emitting diodes with small difference among the individual light emitting diodes can be obtained.
In the epitaxial wafer of the present invention, the compound semiconductor single crystal substrate and the epitaxial layer more preferably have a total thickness of 200 xcexcm or less.
If a light emitting diode is produced by using an epitaxial wafer having such a thickness, a light emitting diode sufficiently satisfying the demand of the production of smaller and thinner electronic products can be obtained.
According to the present invention, for example, the compound semiconductor single crystal substrate is composed of gallium phosphide, both conductivity type of the compound semiconductor single crystal substrate and conductivity type of the buffer layer are n-type, and the light emitting region is preferably doped with nitrogen as radiative center.
By utilizing the epitaxial wafer of the present invention having such characteristics, light emitting diodes for display such as those for indicator lamps and numerical indication devices can be produced, and light emitting diodes exhibiting green light emission around a wavelength of 570 nm, to which human eyes show the highest visibility, can be obtained.
The light emitting diode of the present invention comprises the aforementioned epitaxial wafer according to the present invention, on which electrodes are formed. in the light emitting diode of the present invention, at least a part of the electrodes formed on the compound semiconductor single crystal substrate side is preferably formed over the exposed area of the epitaxial layer.
According to the present invention, there can be provided an ultra thin type light emitting diode where generation of ohmic electrode failure was suppressed, and an epitaxial wafer for the light emitting diode. Accordingly, the present invention can sufficiently satisfy future demand for thinner light emitting diodes, and can sufficiently cope with the production of smaller and thinner electronic products.